A communication system includes a Downlink that conveys signals from transmission points such as Base Stations (BSs) to User Equipment (UEs) and an Uplink that conveys signals from UEs to BSs.
When an active antenna array, or active antenna system (AAS), is used in cellular wireless communications, the AAS can shape or focus radio frequency (RF) energy in the downlink, and receive sensitivity in the uplink, by adjusting the magnitudes and the phase shifts of the transmit and receive signals at its plurality of antenna elements. In cellular systems, downlink refers to the transmit (TX) operation of the AAS, and uplink refers to receive (RX) operation.
Existing codebook designs for UE feedback are not suitable for operation with active antenna systems capable of producing narrow beams. In addition, such codebooks have been designed keeping in mind point-to-point MIMO (also known as single-user MIMO (SU-MIMO)) with very limited support for the possibility of transmitting to multiple UEs at the same time known as Multi-User MIMO. One object of the concepts described herein is to enable MU-MIMO transmission to UEs with active antenna systems using the existing codebook designs.